Health and environmental concerns have prompted a shift towards the use of latex paints instead of oil-based paints. Latex paints are challenging to formulate for many reasons, such as compatibility issues, drying problems, syneresis issues, and inferior physical properties. Latex paints tend to have softer films when they are dried since they do not react or crosslink like older oil based (alkyd) paints. In particular, latex paints have difficulty in maintaining consistent color when a painted surface is cleaned or wiped to remove a smudge or stain on the wall. Often some color is removed with the wiping material and a noticeable color change is effected (referred to as the “color rub-off”).
Another problem with latex paints is a need to prevent water stains from showing on a painted surface (referred to as the “water staining” issue). Water stains occur when a painted surface is either washed to remove a stain or water is applied, condensed or spilled on a painted surface (such as near a shower) and the water leaves a visible mark on the painted surface after it evaporates, usually due to some surfactant being dissolved in the water and left as a residue when the water evaporates.
In the prior art, if more colorants are used to obtain deeper colors or improved hiding, more surfactants are also added to the paint through the colorants. Colorants typically include large amounts of surfactants to improve pigment wetting and compatibility with the wide variety of tint bases in which they are used. However, the increased surfactants make the paints water sensitive, causing increased water staining and color rub-off.
Color rub-off resistance may be improved with hydrophobic binders (e.g. high styrene content polymers), but these polymers have poor washability for oily materials (“oil stain release”), making them undesirable in a kitchen setting, for example. Sometimes acid monomers (e.g. acrylic acid and methacrylic acid) are used to improve this quality, but they cause reduced resistance to water staining and color rub-off.
Color rub-off resistance may also be improved with hydrophobic solid polymer additives, such as silicone modified urethane or alkyd dispersions. However, there is an incompatibility with acrylic, vinyl acrylic and styrenated polymers, which weaken the film integrity and resistance to washing (as measured by “scrubs”) of the paints. Further, the water staining is still an issue.